1. Field of the Invention
The present invention relates to a process for preparing polyprenols. The polyprenols prepared by the present invention are useful as pharmaceuticals or synthesis intermediates therefor. Among the polyprenols prepared by the present invention, is, for example, 3,7,11,15,19,23,27-heptamethyl-6,10,14,18,22,26-octacosahexaen-1-ol.
2. Description of the Related Art
Several methods are known for reducing an allylic terminal, including:
(1) using NaBH.sub.4 in dimethylsulfoxide as a solvent [see J. Am. Chem. Soc., 92, 4463 (1970)], PA1 (2) using LiAlH.sub.4 [see J. Am. Chem. Soc., 95, 553 (1973)], PA1 (3) using sodium formate as a hydrogen source in the presence of a palladium catalyst [see Chem. Lett., 1463 (1986)], and PA1 (4) using diisobutyl aluminum hydride (DIBAL) as a hydrogen source in the presence of a palladium catalyst [see J. Org. Chem., 56, 2918 (1991)]. PA1 R represents a hydrogen atom or a protective group for a hydroxyl group; and PA1 n is 0 or an integer not less than 1, PA1 which comprises reacting an organic complex of an alkali metal with a compound (hereafter abbreviated as Compound (2)) represented by the following formula (2): ##STR4## where either V represents a halogen atom, while W and X are coupled together to form a carbon-carbon bond, or PA1 X represents a halogen atom, while V and W are coupled together to form a carbon-carbon bond; PA1 A represents a hydroxyl protecting group; and PA1 Y, Z and n have the same meanings as defined above with respect to formula (1).
In the case of the synthesis of a polyprenol from a terminal allyl halide, it is necessary not only to conduct dehalogenation but also to form a carbon-carbon double bond at a selected position. Upon dehalogenation of a terminal allyl halide, use of method (1) described above, however, causes hydroboration owing to BH.sub.3 formed after dehalogenation. The more carbon-carbon double bonds present in the molecule, the more eminent such a tendency becomes, which brings about a reduction in the yield of a polyprenol. In the above-described method (2), on the other hand, it is difficult from an industrial viewpoint to handle LiAlH.sub.4 because it is a powder and has undesirable ignition properties. The above-described method (3) involves a problem in selectivity because many positional isomers of the carbon-carbon double bond may be formed. Method (4) described above has high positional selectivity for a carbon-carbon double bond, but requires temperature as low as -78.degree. C. as necessary for the reaction and, in addition, the palladium catalyst is expensive.
Accordingly, any one of the above methods are not satisfactory for the preparation of a polyprenol selectively and industrially advantageously.